1. Field of the Invention
The present invention relates to a photoreceptor and an image formation method, an image forming apparatus and a process cartridge using the photoreceptor.
2. Discussion of the Background
Recently, organic photoconductors (photoreceptors) have been used in place of inorganic photoreceptors for a photocopier, a facsimile machine, a laser printer and a multifunctional device thereof in light of performances and advantages, such as, (a) optical characteristics, for example, width of the range of optical absorption wavelength and size of the amount of absorption of light; (b) electric characteristics, for example, high sensitivity and stable chargeability; (c) a wide selection of materials; (d) ease of manufacturing; (e) inexpensiveness cost; and (f) toxic-free property.
In addition, demand for the size reduction of an image forming apparatus accelerates the size reduction of an image bearing member (photoreceptor). Also, high speed performance and maintenance-free performance have been demanded. Therefore, an image bearing member having high durability has been desired. From this point of view, an organic photoconductor is soft in general and easy to wear down because the surface layer thereof is mainly made of a low molecular weight charge transport material and an inert polymer. When such an organic photoconductor is repetitively used in the electrophotography process, the organic photoconductor tends to be abraded under mechanical stress by a developing system or a cleaning system. In addition, in accordance with demand for the size reduction of toner particles to improve the quality of images, the rubber of a cleaning blade is hardened and the contact pressure between an image bearing member and a cleaning blade is increased to improve the cleaning performance. This accelerates the abrasion of an image bearing member. Such abrasion of an image bearing member causes deterioration of electric characteristics, for example, the sensitivity and the chargeability, resulting in abnormal images, for example, deterioration of image density and the background fouling. When an image bearing member is locally damaged by abrasion, the damaged portion causes streaks on an image resulting from bad cleaning performance on the image bearing member. Currently, this abrasion or damage is a controlling factor of the lifetime of an image bearing member and once an image bearing member has such abrasion or damage, the image bearing member must be replaced immediately to sustain image quality and performance.
Reducing the amount of abrasion described above is desired to obtain an organic photoconductor having a high durability. This is an imminent issue to be solved in this field.
As a technology to improve the anti-abrasion property of an image bearing member, for example, (1): unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637 describes a technology in which a curing binder is used for a surface layer; (2): JOP S64-1728 describes a technology in which a polymer charge transport material is used; and (3) JOP H4-281461 describes a technology in which an inorganic filler is dispersed in a surface layer. Among these technologies, with regard to the curing binder of (1), the residual voltage tends to rise due to bad compatibility between the curing binder and a charge transport material and remaining impurities, for example, a polymerization initiator or non-reacted groups, which results in reduction in image density. The polymer charge transport material of (2) or the dispersed inorganic filler of (3) improves the anti-abrasion property of an organic photoconductor in some degree but not sufficiently to the level required for an organic photoconductor. Furthermore, the residual voltage rises due to the trap present on the surface of the inorganic filler in the case of (3), which tends to cause a decrease in image density. Consequently, the technologies of (1) to (3) do not sufficiently satisfy the total durability including electric durability and mechanical durability required for an organic photoconductor.
Furthermore, to improve the anti-abrasion property and anti-damage property of the organic photoconductor described in (1), Japanese Patent No. (hereinafter referred to as JP) 3262488 describes an organic photoconductor containing an acrylate monomer cured compound having multiple functional groups. Although there is a description that the surface layer provided on the photosensitive layer contains the acrylate monomer cured compound, there is no specific description about a charge transport material but just a description that a charge transport material can be contained in the surface layer. In addition, when a charge transport material having a low molecular weight is simply contained, a problem of the compatibility between the cured compound and the charge transport material arises. This problem causes precipitation of a transport material having a low molecular weight and white turbidity phenomenon, which may result in deterioration of the mechanical strength of the organic photoconductor.
Furthermore, this organic photoconductor is manufactured by reacting the monomer in the state in which a polymer binder is contained so that the curing reaction is not sufficiently conducted. In addition, the compatibility between the cured material and the binder resin is bad and therefore, the phase separation tends to occur during the curing reaction and lead to formation of a rough surface, which leads to bad cleaning performance.
As the anti-abrasion technology for a photosensitive layer in place of these technologies, for example, JP 3194392 describes a charge transport layer manufactured by using a liquid application formed by a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin contains a binder resin having a carbon-carbon double bond and a binder resin having no carbon-carbon double bond. That is, the binder resin having a carbon-carbon double bond reacts with the charge transport material but the binder resin having no carbon-carbon double bond does not react with the charge transport material. It is notable that this organic photoconductor has an anti-abrasion property and electric characteristics in a good combination. However, there is a tendency that when the binder non-reactive with the charge transport material is used, the compatibility between the binder resin and the cured material obtained by the reaction between the monomer mentioned above and the charge transport material is bad and therefore, phase separation tends to occur during cross-linking and leads to formation of a rough surface, which results in bad cleaning performance.
In addition, as described above, the binder resin prevents curing of the monomer and since the monomers specified in JP 3194392 have only two functional groups, the density of the cross-linking is not sufficient. Therefore, the anti-abrasion property obtained in this case is still insufficient.
In addition, even when the binder is reactive with the transport material, the number of the functional groups contained in the monomer and the binder resin is small. Therefore, it is difficult to have a good combination of the combined amount of the charge transport material and the cross-linking density and thus, the electric characteristics and anti-abrasion property are not sufficient.
For example, JOP 2000-66425 describes a photosensitive layer containing a compound cured from a positive hole transport material having at least two chain reaction polymerizable functional groups in a molecule.
However, this photosensitive layer contains the bulky positive hole transport material having at least two chain reaction polymerizable functional groups so that the cured compound has distortion and thus the internal stress is strong. Therefore, the surface of the photosensitive layer tends to be rough and cracking easily occurs over time, meaning that the surface does not have a sufficient durability.
In addition, when an anti-abrasion property of a photoreceptor is improved but causes bad cleaning performance, production of abnormal images with image blur, etc. and image quality deterioration, it can hardly be said that the photoreceptor has a good durability. Especially, when a polymerization or spherical toner which is popular in this technology field is removed from a photoreceptor having a high surface energy (or a high friction index), the toner revolves and remains between the photoreceptor and the cleaning blade and slips therethrough, resulting in bad cleaning performance.
Methods of adding various kinds of lubricants to the surface layer of a photoreceptor have been used and known to reduce the surface energy and the friction index of the surface of the photoreceptor. For example, a method is known in which a lubricant such as a fluorine modified silicone oil is contained in a surface layer. This method is effective to improve cleanability and remove impurities by reducing the surface energy of a photoreceptor. However, this fluorine modified silicone oil moves close to the surface in the process of forming the protective layer and therefore is lost from the surface layer during repeated use. Resultantly, the effect is lost in the early stage due to an extremely small amount of abrasion of the surface layer. Thus, actually, the technology does not sufficiently improve the durability of a photoreceptor.
Various kinds of methods have been tried to add lubricant particulates to the surface layer of a photoreceptor. For example, silicon resin particulates, fluorine containing resin particulates (for example, JOP S63-65449) or melamine resin particulates (JOP S60-177349) can be added. Furthermore, there are methods which describe containing particulates or powder such as polyethylene powder fluorine containing resin powder (JOP H02-143257), fluorine resin (JOP H02-144550), silicone particulates (JOPs H07-128872 and H10-254160) or cross-linking type organic particulates (JOP 2000-010322 and U.S. Pat. No. 5,998,0772) in the surface layer.
Additionally, there is a method of containing methyl siloxane resin particulates in a surface layer (JOP H08-190213). The methods in which these lubricant particulates are dispersed in the surface layer of a photoreceptor are effective in terms of increasing stability of the effect in comparison with the method of adding silicone oil.
However, since the lubricant is contained in the charge transport layer having an insufficient abrasion property, the effect of restraining the attachment of various kinds of materials at initial stage does not continue for an extended period of time.
Furthermore, a method in which acryl modified polyorganosiloxane having compatibility with a binder resin is contained in the surface layer (JOPs 2005-208112 and 2006-17949) is proposed and effective in terms of reduction in the surface energy, improvement on the anti-abrasion property and cleaning property and restraint of occurrence of image blur. However, the cleaning blade is continuously made in touch with the photoreceptor and tends to be broken prior to the photoreceptor, which has an adverse impact on the working life of a process cartridge.
As described above, currently the typical photoreceptors having a cross-linking photosensitive layer containing a lubricant do not have sufficient property in total.